Interaction of slow highly-charged ions with metals and insulators

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Abstract

Interaction of slow highly charged ions with insulator as well as metallic surfaces is discussed. In addition to the usual flat surface targets, studies with thin foils having a multitude of straight holes of ∼100 nm in diameter (micro-capillary foil) are introduced, which provide various unique information on the above surface interaction. In the case of an insulator micro-capillary foil, a so-called guiding effect was observed, where slow highly charged ions can transmit through the capillary tunnel keeping their initial charge state even when the capillary axis is tilted against the incident beam. A similar guiding effect has recently been found for slow highly-charged ions transmitted through a single tapered glass capillary. In both cases, the guiding effects are expected to be governed by a self-organized charging and discharging of the inner-wall of the insulator capillary. One of the prominent features of this guiding effect with the tapered capillary is the formation of a nano-size beam, which can be applied in various fields of science including surface nano-modification/analysis, nano-surgery of living cells, etc.

Introduction

Interaction of slow highly-charged ions (HCIs) with matter has attracted much attention for decades because of their strong electric fields and large potential energies [1], [2], [3], [4], [5]. When an HCI approaches a surface, target valence electrons are resonantly transferred to highly excited states of the HCI. According to the classical over barrier (COB) model, the distance dc for the resonant charge transfer to start is given by 2q/W and the principal quantum number nc, into which an electron is transferred, is given by ncq/2W(1+q/8), where q is the charge of the incident ion and W is the work function of the target [4]. Such an atom (ion) in multiply- and highly-excited states with inner-shell vacancies formed above the surface will be referred to as a hollow atom in the first generation (HA1). When the ion further approaches the surface, charge transfer channels to lower excited states open and some of the electrons already in highly excited states are released to vacuum or returned to empty levels of the target. In other words, the HA1 is in a non-stationary state forming a molecular orbital with the target, which may more suitably be called a “dynamic hollow molecule”. Because the HCI is accelerated toward the surface due to its image charge, the time interval between the HA1 formation and its arrival at the surface is limited to less than ∼10−14–10−13 s, which is possibly shorter than its intrinsic lifetime. At or below the surface, the valence electrons of the target dynamically screen the ion charge, and considerably promote the energy levels, which allows some shallow inner-shells to be filled via quasi-resonant charge transfers or Auger transitions. If the incident ion has holes at levels deeper than these shallow inner-shells, a hollow atom (ion) can still survive (HA2: a hollow atom in the second generation). The HA2 cascades down to its ground state emitting X-rays and/or Auger electrons releasing its potential energy. Fig. 1 schematically shows how these processes evolve in front of the target surface.

When the same phenomena are viewed from the target side, target atoms near the injection point of the HCI are firstly stripped off their electrons and are then further excited or ionized by Auger electrons and also X-rays emitted from the HCI followed by neighboring electrons re-filling vacancies. Because of this dynamic energy exchange near the injection point of the HCI, a “hot spot” is formed. Atoms and/or ions are often emitted (potential sputtering) [6], [7] leaving sometimes a nano-crater [8]. Depending on the electronic and thermal conductivities, the sublimation energy, etc., its lattice structure may also seriously be damaged/deformed resulting in a nano-dot structure [9]. Some of them are more related to the below surface interaction of slow HCIs with the target. As an example of nano-craters formed by slow HCIs, an STM image of the self-assembled monolayer of thiol bombarded by 1.6 keV Ar8+ is shown in Fig. 2[8].

Slow HCI-surface interaction had already been studied in 1973, more than twenty years ago, measuring secondary electron yields for various ions [1]. X-ray emission phenomena in collision of slow Ar17+ ions with a Be target were studied with a Ge detector in 1985, which indicated, that, e.g. 2p–1s X-rays were emitted after the L-shell was at least partially filled [2]. In 1990, a similar system (Ar17+ + Ag) was again studied but with a crystal spectrometer having much higher resolution than semiconductor detectors [3], which revealed that some X-rays are emitted from a state with a completely filled L-shell and one K-shell hole. Projectile Auger electrons were intensively measured trying to find electrons from HA1 [10]. However, it is often the case that signals from HA2s are much stronger than those from HA1s and the survival times of HA1s are possibly too short for their intrinsic and stationary nature to be studied.

In order to exclusively study the above surface interaction, a thin foil with a multitude of straight holes of ∼100 nm in diameter (micro-capillary foil) was adopted as a target [11], [12]. HCIs passing through the small capillary without suffering violent collisions with the capillary wall can still capture multiple electrons into highly excited Rydberg states, i.e. hollow atoms (ions) are extracted from the capillary and their characteristic features such as the electronic states and their decay modes are directly studied in vacuum. Current interests in HCI-surface studies shift more to insulator targets as well as below-surface phenomena, where nano-modification of the surface and its application are the important subjects.

In the following sections, a brief description is given for a conductive multi-capillary and then recent progresses on insulator targets (multi- and single-capillary) are discussed. It was found that a self-organized charge up plays an essential role in the interaction between charged particles and an insulator. A tapered glass capillary has the ability to form nano-beams of various charged particles having various kinetic energies. Once a nano-beam of HCIs is available, patterning of nano-dots/craters and element-sensitive nano-imaging become possible.

Section snippets

Conductive multi-capillaries and beam capillary spectroscopy

As discussed in the introduction, thin foils with straight multi-micro-capillaries enable to preferentially observe HA1s. When HCIs enter the capillary along its axis, they are attracted toward the inner-wall by the image force and part of them approach the capillary wall within a distance less than dc at a certain depth from the entrance surface and are expected to capture electrons resonantly into their excited states, i.e. HA1s are formed as in the case of flat surface targets. If the

Insulating multi-capillaries and guiding effects

The combination of charged particles with insulator targets has not been studied systematically because of macroscopic charge up induced on the target surface. Observed results depend on the initial surface conditions, the incident current, even the target shape, etc. which do not allow making consistent analyses and predictions.

In order to get physically meaningful results in studying the interaction of charged particles with insulator targets, one of the following three conditions may need to

Single insulator capillary and nano-beam

As an interesting extension of the guiding effect, a single tapered glass capillary has recently been studied [26]. Like in the case of insulator multi-capillary, HCIs were guided keeping the initial charge. The guiding angle was the same as the tilting angle within the experimental accuracy. The prominent feature of this guiding effect with the single tapered capillary is the formation of a nano-size beam, which can be applied in various fields of science.

Fig. 6 shows a schematic diagram of

A new application: end-closed single capillary

Nebiki et al. [28] reported that a 2 MeV He+ ion beam was guided through a tapered glass capillary with inlet diameters of about 1 mm and outlet diameters in the submicron range. It was further indicated that the extracted beam density could also be considerably enhanced (focusing effect). Although the detailed mechanisms of the focusing effect has not yet been studied in detail, the experimental results accumulated till now indicate that the enhancement may come primarily from small angle

Summary

The interaction of charged particles with various type of materials is briefly discussed paying particular attention to insulator targets, which are found to be not chaotic but fairly systematic, where a self-organized charge up plays a critical role. The guiding effect which has been known for insulator multi-capillaries is found to exist also for single tapered glass capillaries, which enables to prepare a nano-beam of slow highly charged ions.

Acknowledgements

The author is deeply indebted to the collaborators, particularly Y. Kanai, T. Ikeda, Y. Iwai N. Stolterfoht, J. Burgdoerfer for their fruitful and vivid discussions. The work is supported by Special Research Projects for Basic Science of RIKEN.

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